Typically, a transparent layer is provided on the data recording face of an optical medium to protect the face. In order to correct spherical aberration caused by differences in thickness among transparent layers, a correction method may be sometimes used in a conventional optical pick up device in which a lens driving means moves a collimation lens to correct a convergence-divergence angle of a light beam entering an objective lens. Another correction method may also be used in which a lens driving means moves a beam expanding lens to correct a convergence-divergence angle of a light beam entering an objective lens.
In the above methods, the lens driving means is in most cases configured in such a way that two guide shafts positioned in a casing of the optical pick up device are inserted into a lens moving unit so that a stepping motor positioned in the casing can control the unit movement (Patent Document 1).
Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-100481 (FIG. 1 through 3)